1. Field of the Invention
The present invention generally relates to an apparatus and method useful for estimating the value of a parameter of a downhole formation. The present invention comprises a particularly useful method for determining the value of a parameter of the formation face being penetrated by the drill bit in realtime during the actual penetration. More particularly, the present invention relates to a method for estimating the value of a parameter of the formation face being penetrated by comparing the value of a measurement-while-penetrating (MWP) parameter of the formation face with prior acquired and correlated data.
2. Description of the Background
The desirability of logging a borehole during or immediately after drilling has long been recognized by those associated with drilling operations. However, borehole logging for many years was exclusively performed by wireline tools lowered into the borehole after removal of the drilling apparatus therefrom. These wireline logging operations, requiring the tripping of the drill string, resulted in lost drilling time and greatly increased costs. Further, changes in the values of various formation parameters occurred during the delay between the actual drilling of a formation and the performance of these wireline logs. For example, leakage of drilling fluids or formation fluids across the boreholes wall during this delay often resulted in the production of inaccurate and incorrect logs. Finally, the results of wireline logging are often not available to the drilling operator and geologist until many hours after a formation has been penetrated.
For many reasons, including those set forth above, those skilled in the art have long recognized the desirability of performing borehole logging operations while drilling. In recent years, there has been significant interest in the development and use of measurement-while-drilling (MWD) systems. However, only recently have appropriate tools and methods to perform logging operations while drilling become available. The actual MWD tools must be resistant to the constantly vibrating drill string and the prolonged exposure to the harsh borehole environment. Further, these tools must be sufficiently strong to withstand the stresses in the drill string and sufficiently small to avoid interference with the operation of the drill string and its associated downhole systems.
Although it is theoretically possible to store the data acquired by MWD tools in a microcomputer or other downhole storage device for transfer to appropriate data processing devices at the surface upon retrieval from the borehole, these systems have not found widespread use. Contemporaneous analysis permits the drilling operator or geologist to immediately detect changes in the near bottom hole conditions and to make any necessary or desirable adjustments in the drilling operation. In order to maximize the benefits of MWD systems, it is necessary to transmit the data immediately to the surface for analysis. Typical telemetry systems include systems for transmitting electrical signals through electrical conductors embedded in or on the drill string, systems for transmitting acoustic signals through the drill string or the drilling fluids and systems for imparting measurable pressure pulses to the drilling fluids.
Although these MWD systems are vast improvements over wireline logging systems, they still suffer from a time lag between the time a new formation face is penetrated and the time the MWD sensors are adjacent the face for measurement. This time lag may be as short as several minutes or as long as several hours. During this time lag, changes may occur at the formation face.
More importantly, the drilling operator and geologist are unaware of the values of the parameters of the formation face actually being penetrated. The MWD data provided to the drilling operator or geologist is characteristic of the formation at the location of the MWD sensors. These sensors are typically located in a drill collar several feet, e.g., ten to fifteen feet, above the drill bit. Accordingly, the drilling operator or geologist is unaware of the values of the parameters at a given location until the borehole has actually progressed to a greater depth so that the MWD sensors are adjacent the given location. The inherent time lag is a function of both the penetration rate and the distance separating the drill bit and the MWD sensors. The time lag is directly proportional to the separation between the drill bit and the MWD sensors and inversely proportional to the rate of penetration. During this lag period, the drilling operator and the geologist are uninformed concerning the values of the parameters of the actual formation face being penetrated.
The advent of MWD technology has decreased the lag time between the time a formation is actually penetrated and the time data characteristic of the formation is available to the drilling operator and geologist. The safety and efficiency of the drilling operation has been improved with this knowledge, permitting evaluation of the formation and modification of the drilling operation as necessary or desirable. However, this analysis and modification is still based upon MWD data obtained as much as several hours after a formation is penetrated. The benefits of MWD information would be maximized if this lag time could be eliminated by providing the drilling operator and geologist with data characteristic of the formation face being penetrated contemporaneously with penetration.
Accordingly, there has been a long felt but unfulfilled need within the borehole logging industry for an apparatus and method useful in providing information concerning the formation face being penetrated contemporaneously with penetration of that face.